1. Field of the Invention
This invention relates to a method and a circuit for stabilizing the gain of raditaion detectors of a radiation detection system. In particular, this invention relates to the gain stabilization of photomultiplier tubes of a gamma scintillation camera.
2. Description of the Prior Art
Radiation detectors are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the nuclear medical diagnosis of a human body organ. A typical radiation detector of a type which the present invention relates is a commercial version of the Anger-type scintillation camera, the basic prinicples of which are described in Anger U.S. Pat. No. 3,011,057.
Such a scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. As individual gamma rays are emitted from the distributed radioactivity in the object and pass through a collimator, they produce scintillation events in a thin planar scintillation crystal. The events are detected by photodetectors positioned behind the crystal. Electronic circuitry translates the outputs of the photodetectors into X and Y coordinate signals which indicate the position in the crystal of each event and a Z signal which indicates generally the energy of the event and is typically used to determine whether the event falls within a preselected energy range (window) for inclusion in the formation of the image. A picture of the radioactivity distribution in the object may be obtained by coupling the X and Y signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals. The detection circuitry typically provides for integrating a large number of spots onto photographic film.
Modern types of scintillation cameras which comprise circuitry for energy and linear spatial distortion correction are described for example in Stoub et al. U.S. Pat. No. 4,298,944, Arseneau U.S. Pat. No. 4,323,977 and Del Medico et al. U.S. Pat. No. 4,316,257. These modern cameras can provide virtually perfect precision of spatial response and uniformity of energy signal response, i.e. spatial linearity and a flat (constant) Z-map.
However, it is well known that in radiation detection systems, the gains of radiation detectors such as photomultiplier tubes in gamma scintillation cameras, change with age, with temperature, with instantaneous current, high voltage conditions, external fields, as well as other less obvious influences, such as voltage and current history. Due to these gain changes, there occurs subsequent tuning errors. If all the photomultiplier tubes in a camera should change in concert, the only effect would be a shift in the net signal amplitudes.
Such an effect could easily be accommodated by a circuitry as described in Miller U.S. Pat. No. 4,296,320 or in the study "Stabilizing Scintillation Spectrometers With Counting-Rate-Difference Feedback" by H. de Waard, Nucleonics Vol. 13, No. 7, July 1955, 36-41. In these cases the analyzer responds to a source of radiation embedded into the scintillation crystal and having a known peak in its energy spectrum. A high voltage regulator is provided, which adjusts the high voltage source of the photomultiplier tube so that the known peak in the energy spectrum corresponds to a desired pulse height.
Similar calibration methods and circuitry are described in the study "Direct Current Stabilization of Scintillation Counters Used With Pulsed Accelerators" by F. P. G. Valckx, Nuclear Instruments and Methods 10 (1961) 234-236, North-Holland Publishing Company; and Peter U.S. Pat. No. 3,903,417. While F. P. G. Valckx makes use of a constant light source, Peter applies to a so-called beta light for calibration.
However, it is never the case that all photomultiplier tubes exhibit gain changes of a uniform nature. Individual photomultiplier tube gain drifts differentially with respect to the average gain drift of the set and can, in short times for some photomultiplier tube types, yield substantial local z-map errors. Such local z-map errors, in turn, result in local sensitivity variations. These variations are mild for on-peak imaging circumstances (photopeak centered in the analyzer window) and measure to be about 1% sensitivity loss per KeV (shift) for the 140 KeV photopeak of 99mTc. Substantially greater sensitivity variations are present for off-peak imaging. These effects can easily be greater than 8% per KeV for a 7 KeV shift of the window to the high side of the 140 KeV photopeak.
Periodic gain calibration, i.e., tuning, service by camera technicians, utilizes a serial tuning algorithm, an example of which is, for instance, described in the European patent application No. 0,023,639. Such an algorithm is iterative. Most factory and field technicians use this approach today in tuning cameras.
However, the major difficulty in the serial tuning algorithm is related to the convolution effects of the photomultiplier tubes within the set. These convolution effects preclude a simple, direct photomultiplier tube gain adjustment in a camera system. Other difficulties are related to the perturbations to overall response due to phototube current changes and scintillator after glow characteristics when compact sources are used to selectively illuminate photomultiplier tubes in the detector head.